In cell phones, digital audio players, etc., a class D amplifier that has high power efficiency and that can be miniaturized is used in more and more devices. In a class D amplifier, differential input signals are compared with a sawtooth or triangular reference signal to generate differential PWM signals and, on the basis of the differential PWM signals, a driver circuit is driven. One drawback of the class D amplifier is that electric power is consumed each time the signal logic of the differential PWM signals is changed.
Consequently, a class BD amplifier, in which the driver circuit is driven on the basis of the phase difference signal of the differential PWM signals, is receiving more attention. In the class BD amplifier, the pulse signal is generated and electric power is consumed only when there is a phase difference between the differential PWM signals. As a result, it is possible to decrease the power consumption in the class BD amplifier relative to the class D amplifier.
In the class D amplifier and the class BD amplifier, the output section of the driver circuit includes a PMOS transistor and an NMOS transistor connected in tandem between the power supply terminal and the ground terminal. If the PMOS transistor and the NMOS transistor are turned on at the same time, a current flows between the power supply terminal and the ground terminal, so that power consumption is undesirably increased.
As a result, a dead time is forcibly set in the differential PWM signals input to the gate of the PMOS and NMOS transistors connected in tandem between the power supply terminal and the ground terminal. When these transistors are switched on/off, they are turned on/off together.
However, for the class BD amplifier, there is the following characteristic feature: on the basis of the phase difference signal between the differential PWM signals, the driver circuit is driven, so that the pulse width of the phase difference signal is intrinsically narrow. Consequently, as the dead time is set, the pulse width of the phase difference signal disappears entirely, so that a normal amplification operation cannot be carried out.